Basic Science II presents another geological subject that also was once the writer's (NMS) principal specialty - the study of nuclear explosion craters and their natural counterparts, meteorite (asteroid) impact craters. For years, impact cratering was esoteric. Now, impacts are hot topics, especially since they have been identified as the likely immediate cause of dinosaur extinction. Where in the 1960s the number of scientists working on impact craters were probably less than 20, now there are hundreds. More to the point, scientists and the general public now realize that impacts by extra-terrestrial bodies (asteroids, comets, meteorites) are genuine dangers to mankind, capable of causing catastrophes greater than any other natural process known to affect the Earth's surface. Plus, they are almost certainly to occur (as they already have) as huge events sometime in the future of civilization. This subject of impacts is pertinent and interesting, and most impact craters are detectable by remote sensing (but newly discovered ones require ground truth and examination of their rocks to prove they are of impact origin). -

Remote sensing has played an important role in the study of impact craters. Since Landsat, and now a bevy of new, higher resolution earth-observing satellites, the entire land surface of Earth has come under surveillance. Previously known impact craters are being re-examined in the context of their surroundings. But, the scientific "fun" has been to search for new ones, especially in isolated regions not easily accessed or populated. The basic strategy in the hunt is to look for distinct circular structures or features, the hallmark of craters, and then to find confirmatory evidence that impact was involved. New craters have been discovered in satellite imagery, as we shall see. Let us start this Section by showing a crater discovered entirely with such imagery, and first reported on in 2010.

This is the small (45 meters), fresh, young (perhaps less than 5000 years old) crater Kamil in the southern Egypt desert. It was first noted in high resolution GeoEye (Digital Globe) imagery but although still with distinctive morphology easily missing unless one were actually perusing such images specifically to find impact craters. The crater is imposed on sandstone, covered by loose sand, that has distinct ejecta blocks. The presence of iron meteorite fragments confirms its impact origin. Here are the "discovery" images and a ground photo showing the team of geologists who went into the wilds to analyze the crater.

There are several reason why both scientists and laypersons have come to know about impacts and their consequences. One is the realization that the Moon and all the Solar System's rocky planets and many of their satellites have circular structures, the majority of which are impact-produced. Impact seems responsible for events in the Earth's past that made significant changes in the evolutionary history of animals and plants. The catastrophic damage done by impacts has become a popular topic for TV and Hollywood movies. Perhaps most compelling is the knowledge that at least one major extraterrestrial object has caused widespread damage in the lifetime of people still living on Earth. The message is: impacts still happen and may do so again.

This was the Tunguska event of 1908 that took place over east-central Siberia. This was manifested by an explosion (equivalent to about 15 megatons of TNT) in the lower atmosphere of an incoming comet or meteorite. This occurred in a then uninhabited part of Siberia. Here is its location:

The explosion was heard by many and barometric gauges over much of the world registered overpressures from compressivive atmospheric waves. Years later explorers reaching the region below the blast found millions of downed trees:

If the ferocity of the Tunguska event didn't get your attention, the graph below should. It shows the energy release caused by asteroidal impacts, expressed in terms of megatons of Hydrogen bomb equivalents, and the likelihood in terms of average number of years between cratering events of different magnitudes (frequency of recurrence):

The writer's interest in impact craters began in 1959 when I joined the Lawrence Radiation Laboratory in Livermore, CA where my job was to support the Nuclear Weapons Division inasmuch as the testing of nuclear devices went underground, so that geological factors now had a major role in controlling and interpreting phenomena associated with the explosions. In April of 1960 I was assigned a most peculiar job: to supervise mining around a 1000 pound detonation within a salt dome outside of Winfield, LA so as to assist the physicists who had calculated the effects of this explosion as a simulation of a contained nuclear detonation. The model developed for the computer run had predicted fractures that would radiate uniformly from the cavity that held the Pelletol (dynamite in pellets) charge. I found this model to be somewhat wrong - the fractures were confined to a narrow sector of about +/- 60 degrees because of the influence of gypsum bands within the salt. These are two illustrations from the first paper I ever had published, about this follow-up study of the Project Cowboy event:

The salt immediately beyond the cavity walls showed numerous small fractures which actually were slip planes that allowed the halite crystals to deform by a quasi-plastic flow. Under a binocular microscope I could see that tiny spheres which were bubbles containing carbon dioxide gas had all been deformed into (strain) ellipsoids whose long axes were all pointed in the same direction, i.e., were aligned. When this next photo was taken, the gas had entirely leaked out but the cloudy appearance of the salt results from the close-spaced slip planes:

I was particularly intrigued by one sample. The salt had been so severely shocked that the halite crystals were partially isotropized but retained residual strain bands:

It was this project and in particular the effects seen in the salt crystals that started my interest in shock effects in rocks that grew into more studies of nuclear and chemical explosions on the rocks themselves and eventually into my major scientific endeavors concerning the rocks involved with meteorite and asteroidal impacts.

(An exceptional summary of impact cratering, and especially the record of these events imposed on the target rocks, is a 120 page treatise written by a colleague, Dr. Bevan M. French, and entitled Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures, published by the Lunar and Planetary Institute [Houston, TX]; LPI Contribution No. 954, 1998.)

The Nature of Craters

After more than 50 years of impact cratering investigations, we may now assuredly say that IMPACT is one of the three fundamental processes involved with planetary and asteroidal bodies (the others being VOLCANISM and CHEMICAL/PETROLOGIC DIFFERENTIATION; on Earth, TECTONICS is a co-equal fourth). Excepting probably Jupiter's Io, whose young surface seems devoid of impact (but not volcanic) craters, all other bodies have varying numbers of craters (Saturn's Titan remains uncertain, and Pluto still hasn't been imaged at resolutions that would detect craters), from the 200+ on Earth to millions on the Moon, Mercury, and most of the satellites of the Giant planet.

Thus, one of the indirect payoffs of the space program is that much of the general public now realizes that fast-moving asteroids, comets, and large meteorites collide with growing or stable planets during their formative period. (Asteroid catastrophe movies are fashionable.) Moreover, impacts persist in modifying such bodies throughout their history. The surfaces of some terrestrial-type planets and many of their satellites display craters that totally pockmark their surfaces: Mercury (top) and Callisto (fourth satellite out from Jupiter) are prime examples.

Earth's Moon has conspicuous craters with generally circular depressions, ranging in size from less than an inch to more than 1,200 km (746 mi) in diameter. Many of the larger ones are visible at full Moon through a pair of binoculars. The most conspicuous lunar impact structure is Tycho (below, on the left) located on the Moon's southern hemisphere. We can readily see that it is the source of great streaks (rays) almost visible to the naked eye that result from deposits of ejecta hurled across the Moon.

The Tycho crater is the classic exemplar of a large impact structure, with these hallmarks: circular raised rim; concentric nest of slumped walls inside this rim; central (uplift) peak; rough, irregular crater floor (here a mix of fragmental ejecta and lava extrusion); and exterior ejecta in hummocky deposits. Typical of farside craters is Goclenius (55 km, 34 mi wide) and several smaller ones (right) as they appeared to Apollo 8 astronauts circling our lunar neighbor. Note their flat interiors filled with mare lavas.

On Earth, circular structures are rare but, as mapping parts of the continents since the early 19th Century led to discoveries of such features, a number had been identified by the middle of the 20th Century. One of particular note is the series of concentric ridges in Mauritania (West Africa) known as the Richat structure (about 40 km outer diameter). The structure was first discovered from space by Gemini 4 astronauts McDivitt and White. This structure results from doming of sedimentary rocks by an igneous intrusion; erosion has left the quartzite ridges as hogbacks (inclined strata that are raised relative to softer rocks between circles) that outline the "bulls-eye" appearance of the structure. Here is a view taken by the ASTER sensor of the Richat structure which briefly was thought to be of impact origin until a visit by R.S. Dietz found field evidence that precluded that possibility and confirmed a purely internal origin. Below it is a perspective oblique view made from an SRTM-Landsat composite to show this remarkably circular basin as it fits next to a mountain range

Other types of circular depressions, craterlike, can be produced by normal geologic processes. A sinkhole is one example. Another depression is caused by the solution of NaCl in domes that reach the surface after diapiric piercing of sedimentary rocks by plastically-flowing salt. This astronaut photo shows two such domes invading the rocks in Melville Island, near Baffin Island, in the Canadian Arctic:

Less than 100 years ago, scientists considered the concept of craters (especially those that remain circular) formed by impacts from meteorites and other extraterrestrial bodies ("bolides"), such as comets, as unrealistic and highly improbable. Several scientists had, by then, suggested that such craters covered the Moon, but the bulk opinion attributed these to volcanic processes. And, indeed, there were depressions in terrestrial volcanic fields that bore sometimes strong resemblance to lunar craters.

One volcanic type that leads to near circularity of some of the resulting craters is the maar crater exemplified below by the Crater Elegante (1.5 km [0.9 mile] diameter) in the Pinacate basaltic field in northwest Mexico (below the Arizona border). This kind of volcanic crater often has a subdued rim. It is formed when lava encounters near surface water which flashes into steam causing rock overhead to be pushed out explosively leaving a depression that is somewhat backfilled by volcanic fragments (and later debris washed in). The gashlike depressions outside the rim are caused by water erosion.

This next example helps to show how impact craters further differ from volcanic craters. The large volcano Nyiragongo in Tanzania, part of the East African Rift system chain of volcanoes, has a large central crater that is nearly perfectly circular. This volcanic crater type differs from impact craters by 1) being atop a built up mountainlike edifice; 2) having steep interior walls (although some slumping can occur); 3) having its layered units, exposed in the walls, largely at low angles (subhorizontal) rather than overturned as is the usual case with impact craters; 4) having a rim that is only slightly raised; and 5) not showing distinctive ejecta (fragments with a large size range up to massive blocks) beyond the rim (some may exist if the volcano has explosive phases, but multiple, piled-up lava flows can extend across the slopes). A sixth criterion is usually a diagnostic separator: impact craters occur mostly in non-volcanic rocks (igneous; sedimentary; metamorphic) that are exposed in the rims, infill, and ejecta blankets (but a form of induced volcanism commonly results from the impact, leading to volcaniclike rocks in the crater floor).

Another example is the elongate caldera on the summit of the raised shieldlike basaltic volcano on Isle Fernandina in the Galapagos Islands. Note the irregularities in its shape and the numerous lava flows emanating from the side of the volcano's slopes.

18-1: Until the space program in the 1960s, debate over the nature of lunar craters raged as a controversy for more than a century. But with close exploration of the Moon and then planets like Mercury and satellites like Callisto disclosed that most planetary surfaces were heavily cratered. The weight of opinion shifted drastically in favor of impact as the dominant process creating the myriads of circular depressions spread widely over these surfaces. Can you develop (deduce) some arguments that support this impact hypothesis? ANSWER

Impact as a lunar and terrestrial process was first suggested by European geoscientists in the early 1900s. The proposal that Meteor Crater in Arizona had an impact origin, opened the possibility of impact as the cause of other, similar, circular features. G.K. Gilbert conceived the idea and then supported it by finding iron meteorites around the crater. This "first among equals" crater, about which we will say much more on page 18-5, is shown below, along with a typical meteorite (the Canyon Diablo find) from the ejecta deposits outside the crater rim:

Work by R. Baldwin and others kept the impact process alive as an alternative but the vast majority of astronomers and geologists were highly skeptical. All this changed in 1960 with the classic study of cratering mechanics at Meteor Crater and several nuclear explosion craters by Eugene M. Shoemaker, which opened up the possibility of impact as the cause of similar circular features, of which more than 50 were then known on Earth.

The critical proof that the impact process has occurred on Earth emerged in the 1960s from the study of the effects of the intense pressure (shock) waves generated during impact on the target rocks at terrestrial craters, with key studies by E.M. Shoemaker, E. Chao, W. von Engelhardt, B.M. French and others. These effects are described under the general term "shock metamorphism" which we will treat in detail on page 18-4 and 18-4a. Suffice for this introduction just to list the main effects: Shatter Cones; Breccias (rocks made up of fragments [clasts] held together by a matrix of powder, smaller fragments, and, in some, quenched melt [glass]); Petrographic features in individual minerals such as Planar Deformation Features (PDFs), Deformation Lamellae, Kink banding, and Thetomorphs; Melt (either in discrete layers [that can be many meters thick] or dispersed with other ejecta components; and Injected Veins (some containing Pseudotachylite).

In 1960, the writer (NMS), began my studies of nuclear and chemical explosion craters as part of my job in the Plowshare program (engineering uses of nuclear explosions) at the Lawrence Livermore Laboratory (LRL) (see page 18-4 for further details). The first underground nuclear explosion with Plowshare ties was the Gnome 10 kiloton detonation in potash salt near Carlsbad, NM. (the [in]famous event that heaved up the desert surface, awakening wintering rattlesnakes that caused a mad scramble by nearby observers). The explosion produced a 70 meter (220 ft) cavity into which the roof partly collapsed; the photo below shows a geologist colleague, Don Rawson (at bottom), inside part of the remaining cavity:

In my work at LRL, I was responsible for determining the changes induced in rocks at the Rainier, Hardhat, Danny Boy, and Sedan nuclear explosion sites. Sedan, in particular, yielded quite interesting results because this cratering event was conducted in alluvium that contained granite, sandstone, and limestone "float" (blocks of rock a few inches to a foot in size, carried in by streams), all of which were subjected to strong shock pressures during the explosion. Sedan at that time was the largest cratering experiment yet conducted. A 100 kiloton nuclear device produced this 372 m (1230 ft) in diameter crater. Shown below are, on top, the ejecta being tossed out of the developing crater at about 10 seconds after the detonation (I was located about 6 km (3.8 miles) east of ground zero, on top of Rainier Mesa [the closest approach of anyone that day], busily snapping color photos that were later helpful in determining the downwind debris history). Below it is the crater itself about a week after the event, when I was allowed to move up to the rim to collect samples:

These underground nuclear explosions generate peak shock pressures of the same magnitudes as are developed during impacts. One would therefore expect to find shock effects in the rocks involved that were similar to those noted at impact sites. And indeed I found that shock metamorphic features in these rocks were identical to corresponding features in impactites (rocks from natural craters). This tie-in led to the argument that only impacts could cause high pressures on the order of those in underground nuclear explosions, as the features produced are never found in rocks associated with volcanism, even the types called "explosive".

Gradually, the idea that impact is one of the fundamental formative processes acting on planets won broad acceptance. In fact, scientists have now proven that planets grow by accretion of infalling materials, with the craters representing the last stages of buildup, as the planets reach their full sizes. The nature of impact cratering is important, yet introductory Geology textbooks still treat the subject poorly.

Heavily cratered planetary bodies, such as those shown above, share the impact markings that appear on their ancient surfaces but that have not been fully demolished or masked by erosion, lava outflow, deposition, or obliterating mountain activity. Cratering on their surfaces was most intense during the last stages of planetary growth, very early in solar system time (beginning about 4.6 billion years ago), through a later period of about one billion years, after which, the flux of objects striking those surfaces dropped off notably. Earth, Venus, and parts of Mars, also profusely cratered at the outset, by contrast now show far fewer craters because of the subsequent destructive processes that erased or covered most of the impact evidence left on the primitive surfaces.

With the return of the first lunar samples from Apollo 11, shock effects in moon rocks were observed, supporting the conclusion that the bulk of lunar craters are the result of impact.

Distribution of Impact Craters on Earth

Earth today, despite its many recyclings of continental and oceanic crust, retains signs of huge impacts imposed in the last two billion years, as well as smaller ones that took place since then even up to historic times. Scientists have now found about 200 surviving craters of definite or probable impact origin. This is well below the estimates of tens of thousands that would be expected if Earth's surface and crust had not experienced such dynamic ruin from plate tectonic action and atmospheric-driven erosion. Also, protection by deep oceanic waters (more than 70% of the planet’s surface) and burial by sediments further account for this deficiency in anticipated numbers. Nevertheless, more craters remain to be discovered, and satellite imagery should be an effective means for conducting a systematic search, as we shall see near the end of this survey.

There are a variety of informative Web Sites on impact cratering; most provide a list of known and probable craters. Calvin Hamilton has prepared a general treatment, supported by selected images of terrestrial and planetary craters, at (Solar System Cratering). Still another explanatory site is this review of impact cratering by Koberl and Sharpe; this article contains a nearly up-to-date geographic distribution map of known craters, plotted by size groups, which is reproduced below. A simple listing of craters, with location, size, and age, can be brought up from the Internet simply by pressing these two sites (active as of March 2005): (1); and (2). Another impact crater data base is maintained by the University of New Brunswick. Surfing the Internet under the topic "impact craters" has led to these interesting sites:ErnstonClaudin; impact structures; TheLivingMoon. The first two are produced by Kord Ernston and Ferran Claudin; The Living Moon site has several links (go to its "Menu") that are appropriate to both this and the next Sections, but the identity of the site's creators is not specified.

This is one of the better maps showing global impact crater distribution:

This next map has the advantage of really making the crater locations and distributions quite obvious by using red dots:

Even these maps are a bit out of date. Latest count of confirmed impact structures worldwide is 196.

18-2: Inspection of the above global distribution maps shows large areas on Earth in which confirmed impact craters are absent or sparse. Offer several explanations for this scarcity. ANSWER

We want to call special attention to the impact structures that occur in Canada by displaying this figure which shows locations and names of some of its craters and astroblemes (erosion scars of former craters). The Precambrian Shield of Canada is well suited to preserving craters as old as 1.7 billion years despite erosion by Pleistocene glaciation. It was the first large region of the Earth's continents to undergo organized search and study of impact structures under the direction of C.S. Beals of the Dominion Observatory and his successors, including Michael Dence. The West Hawk Lake structure was the one this writer (NMS) studied in detail through a NASA Research grant. Here is a well-known map, about 30 years out of date, that brought attention to the craters that had been found by 1975 through a systematic, deliberate search of the Canadian Shield:

Since this map was drawn 15 more craters have been found in and around the Canadian Shield.

The Importance of Impact Cratering on Earth and Elsewhere

So, why is the study of impact craters, and the mechanisms of cratering (next page) important? Because over the last 40 years scientists have come to realize that this mode of crater formation is one of the three or four most fundamental natural processes affecting the Earth's (and planets' in general) surface. This will become obvious to you as the next Section - on the planets - is visited. The Moon, Mercury, Venus, and Mars, and the satellites of the Giant Planets, are all thoroughly peppered with impact craters. Earth has around 200 known impact craters but once had many more; active degradational geologic processes (stream erosion; tectonic deformation; burial by sediments, glaciation) that have erased most of the earlier ones.

In the last 100 years, several other meteorite impacts leading to small craters have occurred on the land surface around the globe; even more frequently incoming bodies have fallen into the oceans, potentially causing tsunamis. On February 12, 1947 an iron meteorite (possibly as large as a million kilograms) struck the ground in the Sikhote Alin mountains of eastern Siberia (near Vladivostok). Remnants of the meteorite are findable in a 1.3 km2 strewn field which also contains small craters.

From studies of terrestrial impact craters in terms of size and age, from planetary surface studies, and from monitoring asteroid and comet distributions, a rather accurate estimate of cratering frequency (size versus occurrence in time) has led to plots like this:

This plot would seem to indicate that a 1 km crater would form somewhere every hundred years (had the meteorite above the Tunguska River hit the ground instead of bursting in the air, such a crater would have resulted). Every million years or so, a 10 km crater is a realistic possibility if land is the target. And in time spans of a few hundred million years, the crater so developed by impactors of tens of kilometers diameter (typical of asteroids) would put so much debris into the atmosphere that mass extinctions of many life forms would result. On page 18-4 we will see that this has apparently happened at least twice in the last half billion years.

So, this last conclusion should have grabbed your attention!! Lets proceed to learn more about impact cratering - a universal process - starting with a review of how such craters are generated.